Revista Mexicana de Astronomı́a y Astrofı́sica, 48, 293–298 (2012)
THE ENVIRONMENTAL DEPENDENCE OF THE FRACTION
OF ‘UNCONVENTIONAL’ GALAXIES: LUMINOUS
LATE-TYPES AND FAINT EARLY-TYPES
Xin-Fa Deng, Bei Yang, Cheng-Hong Luo, Xiao-Xia Qian, and Ying-Ping Ding
School of Science, Nanchang University, Jiangxi, China
Received 2012 June 7; accepted 2012 August 13
© Copyright 2012: Instituto de Astronomía, Universidad Nacional Autónoma de México
RESUMEN
Usamos la muestra principal, limitada en volumen, del Sloan Digital Sky Survey Data Release 7 (SDSS DR7), para explorar la dependencia ambiental de la
fracción de galaxias ‘no convencionales’, esto es, galaxias luminosas de tipo tardı́o,
y galaxias débiles de tipo temprano. Encontramos que la fracción de galaxias tempranas débiles aumenta significativamente al aumentar la densidad local, y que la
fracción de galaxias tardı́as luminosas decrece un poco al aumentar la densidad local. Esto muestra que existe una dependencia ambiental de la morfologı́a, además
de la luminosidad.
ABSTRACT
Using the volume-limited main galaxy sample constructed from the Sloan
Digital Sky Survey Data Release 7 (SDSS DR7), we have explored the environmental dependence of the fraction of ‘unconventional’ galaxies: luminous late-types
and faint early-types. It is found that the fraction of faint early-types increases
substantially with increasing local density, and that the fraction of luminous latetypes weakly decreases with increasing local density. This shows that there is an
environmental dependence for morphology beyond that for luminosity.
Key Words: galaxies: fundamental parameters — galaxies: statistics
1. INTRODUCTION
It has been known for a long time that highluminosity galaxies are preferentially “early -types”
and redder (e.g., Bower, Lucey, & Ellis 1992; Strateva et al. 2001; Blanton et al. 2003; Baldry et al.
2004; Balogh et al. 2004; Kelm, Focardi, & Sorrentino 2005), which is correct for the majority of
galaxies. But the differences between the luminous
and early-type fractions imply the existence of substantial populations of ‘unconventional’ galaxies: luminous late-types and faint early-types. The galaxy
samples can also be classified by other galaxy properties, for example, color and morphologies. Similarly, the majority of the red population corresponds
to early-type galaxies, and the majority of the blue
population corresponds to late-types. Thus, red latetypes and blue early-types also are defined as ‘unconventional’ populations. The majority of galaxies
show concordant classification, being either luminous, red, early-type and passive galaxies or faint,
blue, late-type and star-forming galaxies. For exam-
ple, Mignoli et al. (2009) found that about 85% of
the galaxies show a fully concordant classification,
being either quiescent, red, bulge-dominated galaxies or star-forming, blue, disk-dominated galaxies.
‘Unconventional’ galaxies are special and rare galaxies in the universe. Although most authors would
like to explore galaxies with concordant classification, the study of ‘unconventional’ galaxies is genuinely interesting. Schawinski et al. (2007) showed
that the fraction of blue early-types declines with increasing local galaxy density. Bamford et al. (2009)
found that the fraction of red spirals rises considerably with increasing local density, and that the fraction of blue early-type galaxies declines substantially
with increasing local density. Using the volumelimited main galaxy sample (Strauss et al. 2002) of
the Sloan Digital Sky Survey Data Release 6 (SDSS
DR6) (Adelman-McCarthy et al. 2008), Deng et al.
(2009c) reached the same conclusions. Mahajan &
Raychaudhury (2009) explored red star-forming and
blue passive galaxies in clusters.
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The purpose of this paper is to explore the environmental dependence of the fraction of ‘unconventional’ galaxies: luminous late-types and faint earlytypes. For this purpose, the selection of local density estimator is a key step. There are many ways to
measure local density, and no one way is more correct than another. Each method has its limitations.
P
Some authors select the projected local density 5
which is computed from the distance to the 5th nearest neighbor within a redshift slice ±1000 km s−1 of
each galaxy (e.g., Goto et al. 2003; Balogh et al.
2004), which is seriously influenced by projection effects. To decrease projection effects, a simple and direct method is to use the three-dimensional density
estimator, which is computed in a comoving sphere
around each galaxy whose radius is the distance to
the 5th nearest galaxy. Unfortunately, the threedimensional density estimator is severely biased by
the peculiar velocity of the galaxy, especially in a
dense region. As indicated as Deng et al. (2008,
2009c), in the volume-limited main galaxy sample,
the influences of biases of the two density measures
on the conclusions are less important. Thus, in this
paper, we only measure the three-dimensional local
density.
Our paper is organized as follows. In § 2, we
describe the data used. The environmental dependence of the fraction of ‘unconventional’ galaxies is
discussed in § 3. Our main results and conclusions
are summarized in § 4.
In calculating the co-moving distance, we use a
cosmological model with a matter density Ω0 = 0.3,
cosmological constant ΩΛ = 0.7, Hubble’s constant
H0 = 100 h km s−1 Mpc−1 with h = 0.7.
2. DATA
Many of survey properties of the SDSS were
discussed in detail in the Early Data Release paper (Stoughton et al. 2002). Like Deng (2010)
did, we use the main galaxy sample (Strauss et
al.
2002) of the SDSS DR7(Abazajian et al.
2009). The data were downloaded from the Catalog Archive Server of the SDSS Data Release
7 by the SDSS SQL Search(with the SDSS flag:
bestPrimtarget&64>0) with high-confidence redshifts (Zwarning 6= 16 and Zstatus 6= 0,1 and redshift
confidence level: zconf>0.95)1 .
From the main galaxy sample of the SDSS DR7,
Deng (2010) constructed two volume-limited samples
above and below the value of Mr∗ found for the overall Schechter fit to the galaxy luminosity function.
The luminous volume-limited main galaxy sample
1 http://www.sdss.org/dr7/.
contains 120362 galaxies at 0.05 ≤ z ≤ 0.102 with
−22.5 ≤ Mr ≤ −20.5. The faint volume-limited
sample includes 33249 galaxies at 0.02 ≤ z ≤ 0.0436
with −20.5 ≤ Mr ≤ −18.5. It is widely accepted
that luminous galaxies tend to reside in the densest regions of the universe, while faint galaxies tend
to reside in low density regions. For example, Park
et al. (1994) noted that high-density regions preferentially include bright galaxies, low density regions
tend to harbor only faint galaxies. Blanton et al.
(2003) showed that the most luminous galaxies tend
to reside in the densest regions of the universe. But
Norberg et al. (2001) and Deng et al. (2009b)
showed that for galaxies fainter than Mr∗ , the change
of the clustering amplitude of galaxies with absolute magnitude or the environmental dependence of
the galaxy luminosity is very weak, while for luminous galaxies it is quite strong. In this work, we
attempt to define galaxies as ‘luminous’ and ‘faint’,
by the environmental dependence of the galaxy luminosity. Because the environmental dependence of the
galaxy luminosity in the faint volume-limited sample
is fairly weak, we only use the luminous volumelimited main galaxy sample constructed by Deng
(2010).
3. ENVIRONMENTAL DEPENDENCE OF THE
FRACTION OF ‘UNCONVENTIONAL’
GALAXIES
A traditional method of morphological classification is to visually inspect the galaxy images according to Hubble’s classification scheme (Sandage
1961). However, the visual inspection procedure is
very labor-intensive. Thus, it is highly desirable to
find automated morphological classification schemes
in order to classify a huge number of galaxies into
early and late types. There are a number of parameters, such as concentration index, color, spectral features, surface brightness profile, structural parameters or some combination of these, that exhibit a
strong correlation with morphological type and can
be used to classify galaxies (e.g., Shimasaku et al.
2001; Strateva et al. 2001; Abraham, van den Bergh,
& Nair 2003; Nakamura et al. 2003; Kauffmann et
al. 2004; Park & Choi 2005; Yamauchi et al. 2005;
Conselice 2006; Sorrentino, Antonuccio, & Rifatto
2006; Scarlata et al. 2007).
R50 and R90 are the radii enclosing 50% and
90% of the Petrosian flux, respectively. The concentration index Ci = R90 /R50 is known to correlate with the morphological type (Morgan 1958; Doi,
Fukugita, & Okamura 1993; Abraham et al. 1994;
Shimasaku et al. 2001; Nakamura et al. 2003; Park
ENVIRONMENTAL DEPENDENCE OF ‘UNCONVENTIONAL’ GALAXIES
0.8
-22
0.6
Fraction
Mr
-22.5
-21.5
295
0.4
0.2
© Copyright 2012: Instituto de Astronomía, Universidad Nacional Autónoma de México
-21
0
-20.5
-20.5
1
2
3
4
Ci
Fig. 1. Distribution in the concentration index vs. luminosity plane of the volume-limited main galaxy sample.
The symbol frequency is 10 (only one out of ten points
is plotted).
& Choi 2005). Shimasaku et al. (2001) showed that
the (inverse) concentration index C = R50 /R90 correlates tightly with the morphological type; this index is useful for automated classification of earlyand late-type galaxies, if one is satisfied with a completeness of ≃70–90%, allowing for a contamination
of ≃15–20%. Shimasaku et al. (2001) also examined
the correlations of visual morphology with a number
of parameters measured by the photometric pipeline,
and found that the concentration index shows the
strongest correlation with visual morphology, and
that a combination of the concentration index with
other parameters, such as surface brightness, color
and asymmetry parameter, does not appreciably enhance the correlation. So, Shimasaku et al. (2001)
concluded that the concentration index is perhaps
the best parameter to be used to classify morphology
of galaxies, which is consistent with the conclusion
obtained by Doi et al. (1993) and Abraham et al.
(1994). Nakamura et al. (2003) separated galaxies
into early and late types according to C < 0.35 and
C > 0.35, which corresponds to a division at S0/a.
When the visually classified sample is taken as the
reference, the early-type and late-type galaxy samples classified by Nakamura et al. (2003) shows an
82% completeness and an 18% contamination from
the opposite sample. Park & Choi (2005) used the
color versus color gradient space as the major mor-
-21
-21.5
-22
-22.5
Mr
Fig. 2. The fraction of early-type galaxies in different
luminosity bins for the volume-limited main galaxy sample.
phology classification tool and the concentration index as an auxiliary parameter. Early-type galaxies
strongly concentrate within a spot centered at (2.82,
−0.04) in the u − r and ∆(g − i) plane, and centered
at (2.82, 0.3) in the u − r color-concentration index
space. In the upper panel of Figure 1 of Park &
Choi (2005), we can note that the concentration index is still a relatively good and simple parameter to
be used to classify the morphology of galaxies; most
of the early-type galaxies have concentration index
C = R50 /R90 < 0.35.
In this paper, we use the concentration index
Ci = R90 /R50 to separate early-type (ci ≥ 2.86)
galaxies from late-type (ci < 2.86) galaxies. Figure 1
shows the distribution in the concentration index vs.
luminosity plane of the volume-limited main galaxy
sample. Figure 2 presents the fraction of early-type
galaxies in different luminosity bins for the volumelimited main galaxy sample. As seen from Figure 2,
the fraction of early-type galaxies rises considerably
with increasing luminosity.
Due to the bimodality of the u − r color distribution, Strateva et al. (2001) developed a divider
(the observed u − r color = 2.22) and classified galaxies above and below this divider as ‘red’ and ‘blue’,
respectively. But so far, there has not been a luminosity divider above and below which galaxies can
be classified as ‘luminous’ and ‘faint’, respectively.
In fact, we must confess that whether a galaxy is
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DENG ET AL.
0.3
Fraction
0.2
© Copyright 2012: Instituto de Astronomía, Universidad Nacional Autónoma de México
0.1
0
-20.5 -20.9 -21.3 -21.7 -22.1 -22.5
Mr
Fig. 3. The luminosity distribution at both extremes of
density for the volume-limited main galaxy sample: red
solid line for the subsample at high density, blue dashed
line for the subsample at low density. The error bars of
blue lines are 1σ Poissonian errors. Error-bars of red lines
are omitted for clarity. The color figure can be viewed
online.
termed ‘luminous’ or ‘faint’ is often relative. In this
study, we intend to give a clear and physical definition of ‘luminous’ or ‘faint’.
For each galaxy, we calculate the threedimensional distance to the 5th nearest galaxy, like
Deng et al. (2008) did. The local three-dimensional
galaxy density is defined as the number of galaxies
(N = 5) within this distance divided by the volume
of the sphere with the radius of the distance to the
5th nearest galaxy. Like Deng et al. (2008), we arrange galaxies in density order from the smallest to
the largest, select about 5% galaxies (6018 galaxies),
and construct two subsamples at both extremes of
the density. We compare the distribution of galaxy
luminosity in the lowest density regime with that in
the densest regime. As seen from Figure 3, the subsample at low density apparently has a higher proportion of faint galaxies (Mr ≥ −21.1) and a lower
proportion of luminous galaxies (Mr ≤ −21.5) than
the subsample at high density; the level of significance is at least 3σ, which is consistent with the
widely accepted conclusion: luminous galaxies exist preferentially in the densest regions of the universe, while faint galaxies are located preferentially
in low density regions. Thus, we define galaxies
with luminosity Mr ≥ −21.1 as ‘faint’, and gala-
xies with luminosity Mr ≤ −21.5 as ‘luminous’. In
our volume-limited sample, the luminous early-type
class contains 14935 galaxies, the luminous late-type
class 11218 galaxies, the faint late-type class 44665
galaxies and the faint early-type class 20604 galaxies.
Figure 4 shows the fraction [n(‘unconventional’
galaxies)/n(all) in each density bin] of ‘unconventional’ galaxies: luminous late-types (a), and faint
early-types (b) as a function of the three-dimensional
local density LD (galaxies Mpc−3 ) for the volumelimited sample. The LD-axis of this figures is logarithmic. We bin the sample in steps of 0.25 over the
range −4 < log10 LD < 0. As seen from Figure 4,
the fraction of faint early-types increases substantially with increasing local density, and the fraction
of luminous late-types weakly decreases with increasing local density.
Table 1 lists the galaxy number of different
classes of two subsamples at both extremes of density
for the volume-limited main galaxy sample. From
Table 1, we can obtain the same conclusion as from
Figure 4. For the concordant classification, being either luminous early-type galaxies or faint late-type
galaxies, the fraction of luminous early-type galaxies
rises considerably with increasing local density, and
the fraction of faint late-type galaxies declines substantially with increasing local density. Table 1 also
shows that the proportions of luminous galaxies and
early-type galaxies in the subsample at high density
are much higher than the ones in the subsample at
low density. This further confirms the widely accepted conclusion: luminous or early -type galaxies
exist preferentially in the densest regions of the universe, while faint or late-type galaxies are located
preferentially in low density regions.
As seen from Table 1, in the subsample at high
density, the majority of the luminous population corresponds to early-type galaxies, but in the subsample at low density this preference does not exist. In
two subsamples at both extremes of density, we note
that the majority of the early-type population does
not correspond to luminous galaxies. At least in this
study, it is difficult to conclude that luminous galaxies are preferentially early-types, and that earlytype galaxies are preferentially luminous, which was
widely accepted in the past. But Table 1 still shows
that faint galaxies are preferentially late-types, and
that late-type galaxies are preferentially faint. In
fact, ‘luminous’ and ‘faint’, as mentioned above, are
relative concepts. The most luminous galaxies in a
sample may likely be fainter than the faintest galaxies in another sample. So, we should accept such
conclusions with caution. But from Figure 2, we can
ENVIRONMENTAL DEPENDENCE OF ‘UNCONVENTIONAL’ GALAXIES
0.16
297
0.3
0.26
Fraction
Fraction
0.12
0.22
0.18
0.08
© Copyright 2012: Instituto de Astronomía, Universidad Nacional Autónoma de México
0.14
0.04
0.1
-4
-3
-2
-1
0
-4
-3
-2
Log10(LD)
Log10(LD)
(a)
(b)
-1
0
Fig. 4. Fraction of ’unconventional’ galaxies: luminous late-types (a) and faint early-types(b) as a function of the
three-dimensional local density of galaxies for the volume-limited main galaxy sample. The error bars are 1σ Poissonian
errors.
TABLE 1
NUMBER OF GALAXIES OF DIFFERENT CLASSES FOR TWO
SUBSAMPLES AT BOTH EXTREMES OF DENSITY
Classes
Luminous early-types
Luminous late-types
Faint late-types
Faint early-types
Subsample at low density
N (%)
376 (6.25)
518 (8.61)
2854 (47.42)
924 (15.35)
conclude that the more luminous a galaxy is, the
more preferentially early-type it is.
Deng et al. (2009c) found that the fraction of red
late-type galaxies rises considerably with increasing
local density, and that the fraction of blue early-type
galaxies declines substantially with increasing local
density. This is in good agreement with the conclusion obtained by other authors (Schawinski et al.
2007; Bamford et al. 2009). Bamford et al. (2009)
claimed that this trend shows that there is an environmental dependence for color beyond that for morphology. As is well-known, red or early-type galaxies
exist preferentially in dense environments. The environmental dependence of galaxy morphology and
color shows that the transformation of galaxies is due
to environmental mechanisms. For example, some
mechanisms have been proposed for the transformation of spiral galaxies into early-types in dense envi-
Subsample at high density
N (%)
1102
508
1863
1162
(18.31)
(8.44)
(30.96)
(19.31)
ronments (Boselli & Gavazzi 2006). Bamford et al.
(2009) believed that the environmental dependence
for color beyond that for morphology shows that a
transition from blue to red in dense environments
is not generally accompanied by a transition from
spiral to early-type, which means that the environmental transformation of galaxies from blue to red
must occur on significantly shorter timescales than
the transformation from spiral to early-type. In this
study, we find that the fraction of faint early-types
increases substantially with increasing local density,
and that the fraction of luminous late-types weakly
decreases with increasing local density. This shows
that there is an environmental dependence for morphology beyond that for luminosity. Deng et al.
(2009a) investigated the dependence of luminosity
and g −r color on environment for the same morphological types, and also found that the dependence of
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DENG ET AL.
luminosity on local environment is mainly due to the
dependence of galaxy morphologies on local environment and the correlation between morphologies and
luminosity.
© Copyright 2012: Instituto de Astronomía, Universidad Nacional Autónoma de México
4. SUMMARY
In this work, we use a volume-limited main
galaxy sample of the SDSS DR7 which contains
120362 galaxies at 0.05 ≤ z ≤ 0.102 with −22.50 ≤
Mr ≤ −20.50, and measure the three-dimensional local density, to explore the environmental dependence
of the fraction of ‘unconventional’ galaxies: luminous
late-types and faint early-types. It is found that the
fraction of faint early-types increases substantially
with increasing local density, and that the fraction of
luminous late-types weakly decreases with increasing
local density. This shows that there is an environmental dependence for morphology beyond that for
luminosity.
We thank the anonymous referee for many useful comments and suggestions. Our study was supported by the National Natural Science Foundation
of China (NSFC, Grant 10863002).
Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National
Aeronautics and Space Administration, the Japanese
Monbukagakusho, the Max Planck Society, and the
Higher Education Funding Council for England. The
SDSSWeb site is http://www.sdss.org.
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions.
The Participating Institutions are the American Museum of Natural History, Astrophysical Institute
Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University
of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation
Group, John Hopkins University, the Joint Institute
for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max
Planck Institute for Astronomy (MPIA), the Max
Planck Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, Univer-
sity of Pittsburgh, University of Portsmouth, Princeton University, the US Naval Observatory, and the
University of Washington.
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Xin-Fa Deng, Bei Yang, Cheng-Hong Luo, Xiao-Xia Qian, and Ying-Ping Ding: School of Science, Nanchang
University, Jiangxi, China, 330031 ([email protected]; [email protected]; [email protected];
[email protected]; [email protected]).